The gunman who shot Rep. Gabrielle Giffords in the head and then unleashed a spray of bullets that killed six and wounded 12 others at a town hall event outside a Tucson, Ariz., grocery store may not have acted alone, authorities announced Saturday night.

"We are not convinced that [the gunman in custody] acted alone, there is some reason to believe he came to this location with another individual, and that individual is involved," Pima County Sheriff Clarence Dupnik said.

Police said a suspect was taken into custody, and Dupnik described the alleged shooter as mentally unstable. Though the sheriff did not name the suspect, he was identified by multiple law enforcement sources as Jared Lee Loughner, 22.

Dupnik declined to provide more information on the second individual who he would only describe as "white" and "in his 50s." Authorities have photographs of the person of interest and are "actively pursuing him," the sheriff said.

The congresswoman was the clear focus of the gunman's assault, Dupnik said.

"He ran through the crowd and when he got to [Giffords] he just started shooting," the sheriff said.

"The anger, the hatred, the bigotry that goes on in this country is getting to be outrageous. And unfortunately Arizona I think is the capital. We are the Mecca for prejudice for prejudice and bigotry," he said.

Dupnik said authorities are also investigating a suspicious package at one of Giffords' field offices in Tucson.

Steven Rayle, who was at the scene, told ABC News that "another man went running off" after the shooting, but wasn't sure if he was involved.

A total of 19 people were hit by gunfire during the shooting, which began just after 10 a.m. Six of the victims have died, according to Dupnik, and the remaining individuals are not believed to be in "life threatening situations."

Five of the victims died on the scene, and 9-year-old Christina Greene died at a nearby hospital.

Those who died and have been identified are 63-year-old U.S. district Judge John Roll , 30-year-old Gabriel Zimmerman (Giffords' director of community outreach), 76-year-old Dorthy Murray, 76-year-old Dorwin Stoddard and 79-year-old Phyllis Scheck, according to the Pima County Sheriff's Department.

Giffords, 40, was shot in the head and was taken to University Medical Center, where she underwent brain surgery and was listed in critical condition.

Dr. Peter Rhee, trauma director at UMC, told reporters that the bullet went through her brain, but he said she was responsive to doctors' commands.

"I am very optimistic about her recovery," Rhee said.

Richard Carmonia, a former U.S. surgeon general who was at the event where the shooting occured, said that Giffords' injuries were "severe" but that there is a "possibility she will survive."

President Barack Obama, speaking at a nationally televised news conference, called the shooting an unspeakable tragedy.

"What Americans do at times of tragedy is to come together and support each other, so at this time I ask all Americans to join me and Michelle in keeping all the victims and their families, including Gabby, in our thoughts and prayers," he said. "We are going to get to the bottom of this and we're going to get through this."

"The suspect is currently in custody, but we don't yet know what provoked this unspeakable act," the president said.